Problems with a conventional needle valve are that it has a narrow flow rate control range, and as it is repeatedly used, it changes gradually, flow rate control gets unstable or the flow rate cannot be reduced down to zero. With this in mind, the inventor has proposed a flow rate control valve that does not undergo any flow rate change even when there are temperature changes so that a flow rate change relative to the angle of rotation remains constant irrespective of position, as disclosed in JP(A) 2006-138399 (Patent Publication 1).
To provide a solution to a problem that as fine foreign matters enter a flow passage through the flow rate control valve described in Patent Publication 1, it causes the flow rate to get noticeably low in a discontinuous manner, the inventor has later put forward a flow rate control valve having a plurality of grooves, as described in JP(A) 2009-162384. FIG. 7 is a longitudinal section of the construction and arrangement of this flow rate control valve. The flow rate control valve comprises a valve body 110, a sleeve 120, a sliding shaft 130, a moving mechanism 140, a knob 150, O-rings 161 to 165, etc.
The knob 150 is operated for control of a flow rate through the flow rate control valve, and fixed to an operating shaft 142 of the moving mechanism 140. The moving mechanism 140 comprises an operating shaft 142 to rotate the shaft by the knob 150 for axial movement of the sliding shaft 130, and a coupling 141 for coupling the operating shaft 142 to the sliding shaft 130 such that the rotation of the operating shaft 142 is not transmitted to it. The operating shaft 142 is in threaded engagement with a threaded portion 110a of the valve body 110 via a threaded portion 143.
The coupling 141 comprises a box nut 141a, a machine screw 141c, a long nut 141d, etc. for coupling of the operating shaft 142 to the sliding shaft 130 having a tapered groove such that the rotation of the operating shaft 142 is not transmitted to the sliding shaft 130. The operating shaft 142 and box nut 141a are fixed to each other as by bonding of threaded portions, so that they cannot rotate relative to each other.
The machine screw 141c has its head held within the box nut 141a, and bonded to the box nut 141a with its leg in threaded engagement with the long nut 141d by way of a through-hole in the box nut 141a. Further, the long nut 141d is press fitted and fixed by an adhesive in a threaded bore in the sliding shaft 130. A washer formed of ethylene fluoride resin having a low friction resistance is interposed between the head of the machine screw 141c and the box nut 141a, and between the box nut 141a and the long nut 141d so that the rotational motion of the box nut 141a in association with the rotation of the operating shaft 142 is less likely to be transmitted to the machine screw 141c. 
The sliding shaft 130 is constructed from a guide 131, a sliding portion 132 and a tapered groove 133. The guide 131 is connected to the machine screw 141c by way of a threaded bore through the long nut 141d located thereon, and provided with two O-ring grooves on its outer periphery so that the outer periphery is axially movable along the inside surface of the valve body 110.
O-rings 161 and 162 for sealing the sliding shaft are attached to the two O-ring grooves, and a shutoff O-ring 163 is mounted on a stepped portion at the middle. The sliding portion 132 is provided with the tapered groove 133 comprising a plurality of parallel grooves for formation of a fluid passage, said grooves having their depths decreasing or increasing gradually along the lengthwise direction.
The sleeve 120 is configured in such a way as to be open at one (top) end of a cylindrical configuration and closed at the other (bottom) end. The sleeve 120 is attached to the inside of the valve body 110 from a direction in opposition to the sliding shaft 130, viz., from below, and a sliding cylindrical surface 121a formed on the inner periphery of a cylindrical sleeve portion 121 is fitted over the sliding shaft 130 such that they are slidable relative to each other in an axial direction.
An outer diameter portion of the sleeve 120 in a longitudinally central position is provided with an O-ring groove, and an O-ring 165 is provided within this O-ring groove to enable the inner peripheral surface of the valve body 110 to have sealing action thereby preventing a fluid leakage from an inlet side to an outlet side. Below the sleeve 120 there is a sleeve cavity 123 formed to take hold of an area where the sliding shaft 130 is slidable. At a lower end of the cavity 123, a shutoff tapered hole 124a and a radially extending sleeve outlet bore 124b in communication with the hole 124a are provided so as to guide a fluid flowing through a fluid passage formed by the tapered groove 33 to an outlet 114 of the body 110.
The fluid passage formed by the tapered groove 133, used herein, is understood to refer to a portion of an opening surface of the tapered groove 133 that is covered and closed by a portion of the overall length of the tapered groove 133 where the sliding cylindrical surface 121a on the upper end side of the sleeve 120 fits in with the sliding portion 132 of the sliding shaft 130.
Between the outer peripheral surface of the sleeve cylindrical portion 121 and the inner peripheral surface of the valve body 110, there is a flow passage space 113 formed so as to guide a fluid flowing from a fluid inlet 111 of the valve body 110 into an inlet 112 to the aforesaid fluid passage formed by the tapered groove 133.
The fluid entering the valve body 110 from the fluid inlet 111 passes from the inlet 112 through the flow passage space 113 between the outer peripheral surface of the sleeve cylindrical portion and the inner peripheral surface of the valve body 110, then through the fluid passage formed by the tapered groove 133 comprising three grooves, then through the tapered hole 124a and sleeve outlet bore 124b, and then goes out of the fluid outlet 114. And as the knob 150 turns or reverses, it causes the sliding shaft 130 to reciprocate axially relative to the valve body 110 with the result that the sectional area of the opening in the tapered groove 133, viz., the groove in the upper end of the sleeve 120 varies for flow rate control.
This flow rate control valve is capable of precise control over a wide range from very minute flow rates to relatively high flow rates. With increasing frequency in use, however, there has been a phenomenon known to an operator, in which activation of the sliding shaft for flow rate control causes a flow rate to change in a direction opposite to the direction of a temporarily predetermined flow rate change. Reverse movement such as a backlash may often give rise to a grave problem: it may disturb the operator although temporal or throw off the blending precision of the fluid to be prepared. While the aforesaid flow rate control valve has a feature of being capable of precise control of very minute flow rates, it is to be noted that the aforesaid flow rate variations have some considerable or adverse influences on such a minute flow rate control.
Automatic control is generally designed such that a flow rate comes close to the desired one by rapid activation of a flow rate control valve. For such behaviors, however, it is required to gain stepwise or incremental control of flow rate changes: it is necessary for the operator to wait until the flow rate is changed a bit to find one position where the flow rate remains stable, and again change the flow rate to find another position where the flow rate remains stable. Alternatively, it may be possible to extend the operation time long enough to have no adverse influence. In either case, some considerable time is taken for flow rate control.